This invention relates to laser surgery apparatus and methods adapted for use, for example, in the monitoring of laser systems used in ophthalmic laser surgery.
Laser systems have been used in ophthalmic surgery for modifying the cornea of the patient. Systems such as shown in U.S. Pat. No. 4,729,372 to L""Esperance contemplate the controlled ablation of the cornea of the patient with a pulsed excimer laser. Operations performed with the system include corneal transplants and keratotomics.
The application of laser light to the cornea may be controlled by spot scanning of the cornea or by the use of masks. As shown in U.S. Pat. No. 5,108,388 to Trokel, the masks may, for example, employ slits or holes. Repeated scanning or pulsing through properly selected masks are employed to reshape or reprofile the curvature of the cornea to treat myopic or hyperopic conditions. The system can also be used, for example, to remove corneal sections for corneal replacements or transplants.
Three types of laser vision correction surgery techniques are known in the art: broad beam, slit scanning and spot scanning. Broad beam systems use a relatively large beam (e.g. 6.0 to 8.0 mm) pulsed at a relatively low pulse rate (e.g. 10 to 50 Hz). The spot delivered to the cornea may be, for example, from xc2xd mm to 8 mm in diameter depending on the iris opening of the system set to various positions in accordance with a treatment sequence for the patient. Spot scanning systems also called xe2x80x9cflying spotxe2x80x9d scanners typically employ reciprocating or rotating optical devices to make a series of overlapping laser shots, that for example, spiral out from the center of the cornea. Spot scanning systems use a relatively small spot (e.g. 1 to 2 mm in diameter). A typical treatment using a spot scanning system may require several thousand shots at 50 to 200 Hz. In a slit scanning laser, the laser beam is focused through a slit in a rotational device. The slit may be gradually enlarged to increase the ablated area on the cornea. Various scanning systems are described, for example, in U.S. Pat. No. 6,136,012 to Chayet et al., which is hereby incorporated by reference.
A system used by applicant for performing ophthalmic laser surgery is shown in FIG. 1. The system includes an Excimer laser 10 such as a COMPex 201 Excimer laser. An optical rail 12 contains optical elements for controlling the laser pulses and delivers spatially modulated pulses to a shuttling device 14, which acts as a selectively positionable turning mirror, for directing the laser pulses to a selected one of the two surgical stations, 16 and 18. The system allows surgery to be performed on one patient while a second patient is readied, and improves the utilization efficiency of the operating room, laser and optical rail.
FIGS. 2(a) and (b) are vertical and horizontal cross-sectional views and ray traces of an optical path which may be used in the system of FIG. 1 to deliver pulses from the laser 10xe2x80x2 to the cornea of the patient at 20. A light beam from the laser is shaped and focused by a series of lenses 22, 24 and 26. A beam homogenizer 28 is located next in the optical path as shown. A spatial modulator 30 provides beam dimensions and orientations in accordance with predetermined treatment parameters appropriate for the surgery required by the patient. The spatial modulator may include a conventional iris and variable, slit mask(s) as well as controls for changing the axis of orientation of the mask(s). These systems are motor driven on command from a treatment computer containing a treatment algorithm into which the treatment parameters have been programmed.
The shuttling turning mirror 32 selectively directs the laser beam to one or the other surgical stations along one of the system arms 34 or 36 shown in FIG. 1. An imaging lens 38 is located in each arm. Pulses from the imaging lens are reflected by end turning mirror 40 toward the target area 42 on the patient""s cornea.
It is important that pulses delivered to the cornea have the appropriate energy to ensure that the reprofiling, cutting or ablation produced is consistent with the prescribed treatment for the patient. Systems of the type shown in FIG. 2 have employed photo detectors selectively positionable in the main optical path of the system at the end turning mirror for the purpose of calibrating or adjusting the energy delivered by the system during a preliminary calibration phase. See U.S. Pat. No. 5,772,656 to Kloptek.
Other control systems have been proposed such as disclosed in U.S. Pat. No. 4,941,093 to Marshall et al., which includes a measurement device to measure the cornea surface profile and a feedback control system to control the laser operation in accordance with the measured and desired profiles. U.S. Pat. No. 5,423,801 to Marshall et al. discloses further control of the laser by a measurement signal from a beam-shaping means and/or cornea while it is exposed to irradiation by the laser. U.S. Pat. No. 4,973,330 to Azema et al. discloses a photo detector associated with a semi-transparent mirror, which is intended to furnish a treatment computer with information relative to the energy of the pulses exiting the laser before the laser beam reaches the controlling device. A laser calibration device is shown in U.S. Pat. No. 5,464,960 to Hall et al. which employs a phantom cornea with superimposed thin films of alternating colors. U.S. Pat. No. 5,984,916 to Lai discloses a surgical laser system with a feedback system for controlling the treatment laser beam.
It is an object of the present invention to provide a more efficient and reliable technique for monitoring laser surgery, including broad beam, slit scanning and spot scanning systems.
It is another object of the present invention to monitor the energy of actual laser pulses used in the ophthalmic laser surgery as they exit the optical rail.
It is another object of the present invention to monitor a sequence of laser pulses of varying beam dimensions and locations used in ophthalmic laser surgery.
It is another object of the present invention to provide a parallel, fail-safe system for detecting discrepancies between a programmed treatment and the laser pulses actually administered to the cornea of the patient.
These and other objects and features will be apparent from the following description of the present invention contained herein.
The present invention relates to methods for laser surgery and particularly for the modification of the cornea of a patient with a laser system in accordance with treatment parameters appropriate for the patient and for continuously verifying that a predetermined sequence of laser pulses of correct energy are being delivered to the cornea of the patient. In practicing the method, pulses of laser light are generated and controlled. The controlled pulses are simultaneously directed to the cornea of the patient and to a photo detector. Advantageously, the system uses a beam splitter for this purpose. The beam splitter is the last optical element in the optical path leading to the cornea of the patient. An output signal of the photo detector is converted into a value representative of the light energy delivered to the cornea of the patient. Alternatively, the photo detector may be a two-dimensional array of photo sensing cells capable of producing signals indicative of the spacial energy distribution of the treatment pulses. Such an array may, for example, be a CCD or CMOS device.
Light energy values may be compared to a reference values derived from system calibration information and from the treatment parameters for the patient. An indication of the performance of the laser system is provided in response to this comparison. When a two-dimensional detector array is used, a histogram may be produced, displayed and stored showing the amount of energy delivered to incremental areas of the cornea over selected time intervals.
In preferred embodiments of the invention, the pulses of laser light are produced by a laser triggered by a triggering signal from a treatment computer. The pulses of laser light may be spatially modulated or scanned responsive to signals from the treatment computer. The treatment computer is programmed with the treatment parameters appropriate for the patient. In this embodiment, the reference values are produced by a monitoring computer separately programmed with the treatment parameters appropriate for the patient. The double entry of treatment parameters helps expose data entry errors in the treatment computer, since such an error will create a discrepancy between the light energy value and the reference value. The comparison may be initiated by the monitoring computer responsive to the laser triggering signal. When the light energy value of a predetermined number of pulses deviates a predetermined amount from the corresponding reference values, the system may produce an alarm signal or shut down the system.
In another preferred embodiment of the present invention, the simultaneous directing of the spatially modulated pulses is performed by beam-splitting the pulses to direct a portion of electromagnetic energy from the pulse to a photo detector. The directed portion of electromagnetic energy of the laser pulse may be directed through an optical baffle to block scatter caused, for example, by fluids splashed on the beam splitter. The directed portion of the pulse may then be converted to fluorescent light which is detected by the photo detector. One or more neutral density filters may be employed to filter the fluorescent light so that the photo detector and associated amplifier are operated in a generally linear response mode across a range of expected incident radiation energies.
The present invention also includes an apparatus for producing a predetermined treatment sequence of laser pulses of predetermined energy and and for monitoring the energy of the pulses as the pulses are being delivered to the patient. Such an apparatus may include an excimer, pulsed laser, and a beam homogenizer and a spatial modulator in the optical path of the laser. First electronic circuitry controls the laser and spatial modulator in accordance with entered data indicative of the predetermined treatment sequence of pulses for the patient. Second electronic circuitry produces reference values indicative of the energy of laser pulses which should be produced by the laser, the reference value being calculated in accordance with separately entered data indicative of the predetermined treatment sequence of pulses for the patient. Advantageously, the first and second electronic circuitry are separate, programmable digital computing devices.
A photo detector produces a monitoring signal related in value to the energy of laser pulses delivered to the patient. Further electronic circuitry compares the monitoring signal with the corresponding reference value calculated by the second electronic means.
As noted above, the delivered laser pulses may be monitored using a beam splitter which is the last optical device in the system optical path leading from the laser to the cornea of the patient. Advantageously, a second beam splitter and a photo detector may be placed at the beginning of the optical rail to monitor laser output directly. This monitoring may be required because the output of the laser may vary from pulse to pulse or drift over the course of a single patient treatment. Advantageously, this additional detector is capable of detecting an energy change of 2% or less from pulse to pulse. Detected changes greater than a selected threshold level may be used to produce a warning signal or to shut down the system.
The foregoing is intended as a convenient summary of this disclosure. However, the scope of the invention intended to be covered is indicated by the patent claims.